Year	Percent
1975	25.13
1976	24.22
1977	24.06
1978	23.64
1979	24.11
1980	23.7
1981	23.72
1982	18.23
1983	18.12
1984	20.19
1985	20.22
1986	21.19
1987	23.03
1988	24.85
1989	26.48
1990	25.25
1991	26.3
1992	26.47
1993	27.22
1994	26.05
1995	26.06
1996	26.59
1997	27.3
1998	25.52
1999	23.42
2000	23.39
2001	23.92
2002	23.86
2003	23.4
2004	23.6
2005	22.95
2006	23.29
2007	23.38
2008	23.54
2009	54.22
2010	22.96
2011	23.37
2012	23.83
2013	23.88
2014	23.49
2015	23.28
2016	23.01
2017	22.74